fix(settings): show "Passkey limit reached" alert (FXA-13680)

[dschom]

Because

• When a user already at the max passkey limit attempted to register another passkey from a stale Settings session (e.g., a tab on a second device that hadn't refreshed), the auth-server correctly returned a 400 with errno 226 ("Maximum number of passkeys reached"), but PagePasskeyAdd ignored error.errno and rendered the generic "System not available. Try again later." alert — misleading users into thinking the service was down.

This pull request

• Inspects error.errno in the PagePasskeyAdd server-error catch branch. Recognized errnos now render their existing localized AuthUiErrors message via getLocalizedErrorMessage (errno 226 → "Passkey limit reached"); unknown errors keep the previous generic-fallback + Sentry capture path.
• Widens the Glean error reason to auth_error_<errno> for known auth-server errors so dashboards can distinguish them from true server_error.
• Adds unit tests covering errno 226 from both beginPasskeyRegistration and completePasskeyRegistration; existing unknown-error tests continue to pass.
• No new FTL strings or backend changes — auth-error-226 = Passkey limit reached and the PASSKEY_LIMIT_REACHED AuthUiErrors entry already existed.

Issue that this pull request solves

Closes: FXA-13680

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: packages/fxa-settings/src/components/Settings/PagePasskeyAdd/index.tsx (server-error branch).
• Suggested review order: source change, then the two new tests in index.test.tsx.
• Risky or complex parts: low risk — the fix is a guarded errno check that delegates to the existing getLocalizedErrorMessage helper used elsewhere in fxa-settings; all unknown errors retain the original behavior.

How to manually verify

1. Locally set PASSKEYS__MAX_PASSKEYS_PER_USER=1 in auth-server config (or hit the real prod limit of 10).
2. On Device A, register a passkey to reach the limit.
3. On Device B with a stale Settings tab, click Create in the Passkeys row.
4. Expect the alert to read "Passkey limit reached" instead of "System not available. Try again later."

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